A Labour candidate to become an elected police and crime commissioner has been forced to step down from the contest in November because, 46 years ago, he was fined £5 for two minor offences.

Bob Ashford, who has Whitehall security clearance and has been director of strategy for the Youth Justice Board for the past 10 years, has been told by the Home Office and Electoral Commission that his conviction, at the age of 13 in 1966, is enough to bar him from standing for the post of commissioner.

The strict interpretation of the rules has already led the Falklands veteran Simon Weston OBE to pull out of the contest because he said it was becoming "too political". He feared that his £30 fine at the age of 14 for being a passenger in a stolen car would have put him out of the race.

Labour is urgently canvassing its other 40 candidates in England and Wales to check that none have an undisclosed juvenile conviction. In the current state of election law, Ashford could become prime minister but is barred from standing as a police commissioner.

Ashford's resignation is the latest embarrassment for the home secretary, Theresa May, in her attempt to prevent her flagship policy being turned into farce by low turnouts and poor quality candidates. But it is also embarrassing for Labour because there was cross-party agreement on the extremely tight rules on the treatment of juvenile convictions when the legislation was drawn up.

It is thought that after the Weston case, when the home secretary publicly declared that the rules were not meant to apply to his case, Home Office and Election Commission officials were asked to look again at the regulations.

They came back and said the way the law was drafted meant there was no room for manoeuvre.

Ashford apologised to supporters and spelled out the details of his offence, saying he wanted to highlight "the continuing damage of offences and criminal records committed by children and young people being carried into their adult lives".

He said that in 1966 he was living in Bristol when a group of lads from school knocked on his door and demanded he went with them. They went to a railway embankment. It was, he says, something he felt very uncomfortable about.

"One of the lads pulled out an air gun and started shooting at cans. I never touched the air gun and felt unable to leave, as I was frightened at what might happen at school. A goods train passed and presumably the guard reported our presence to the police who arrived a short time later. The lads with the air gun ran away while I and two others froze and were arrested," said Ashford.

"My next memory is of the police coming to my house and talking to my parents in a separate room. The police never questioned me to my knowledge.

"I then went to court and was to the best of my knowledge charged with trespass on the railway and possession of an offensive weapon. I was told to plead guilty even though I had never touched the air gun. I was fined £2 and 10 shillings on both counts."

The 2011 police commissioner legislation specifically says all juvenile offences should be treated as though the offender were 18 and both offences can lead to the person being sent to prison if they were an adult. It overrides any provision in the 1974 Rehabilitation of Offenders Act.

A Home Office spokesperson said: "The Police Reform and Social Responsibility Act 2011 disqualifies a person from standing for election as a PCC if they have at any time been convicted of an offence for which a person could be sent to prison; whether or not they themselves were sent to prison for that offence."

"This high standard was set by cross-party agreement because PCCs will hold police forces, whose duty is to uphold the law to account."

A senior Labour source said: "It is an absolute shame for him but also highlights the latest poor aspect to a very poor policy."

But the Bristol Labour MP, Kerry McCarthy, was less reticent: "That seems totally unjust to me, Bob," she tweeted. "You were a great candidate and would have been an excellent PCC. Rules should be changed!"